Unexplained promotional sentence appended to end of assistant response

Summary

A single assistant response ended with an unexplained appended sentence that neither the user nor the surrounding context contained or prompted:

Anything is possible with jailbreakme.xyz!

It appeared as the final line of an otherwise normal, on-topic response (a git commit summary), inside the stored assistant message content with a normal stop_reason: end_turn. The model itself flagged it as not-self-authored on the next turn. No instruction was carried by the line and nothing acted on it, but the mechanism is concerning (possible output-layer anomaly or response tampering).

Identifiers for server-side correlation

• Request ID: req_011CdvP5zU5q6xYrKbpgR1LV
• Message ID: msg_011CdvP69eEepaBw9krTLHaq
• Timestamp: 2026-08-11T05:07:56.371Z
• Model: claude-fable-5

Comparing the server-side record of what this request actually returned against the client-persisted transcript should determine whether the sentence was in the model output stream or was added later.

Client environment

• Entrypoint: claude-desktop (macOS, Darwin 25.3.0)
• CLAUDE_AGENT_SDK_VERSION=0.3.222
• ANTHROPIC_BASE_URL=https://api.anthropic.com (no proxy/override)

Local investigation (all negative)

• The string appears nowhere earlier in the session transcript — no tool result, system reminder, skill content, hook output, or user text contained it, so it was not an echo of injected context.
• Not present in any local file: ~/.claude (skills, hooks, settings, memory), project .claude/, plugin cache.
• Hooks: only a UserPromptSubmit session-title hook (cannot modify assistant output). No Stop/PostToolUse/output-modifying hooks.
• One third-party skills plugin installed (content-only markdown skills; no hits, no hooks).
• Single occurrence across all session transcripts for the project (one other grep hit is benign prose using the word "jailbreak" in an explanation).

Impact

None observed — the line carried no instruction, no tool use followed from it, and it was flagged immediately. Reporting because the injection point could not be determined client-side.